A zoom generally possesses a plurality of optical subassemblies called “elements”, certain of which are fixed and others of which are movable along the optical axis of the system. A conventional zoom architecture comprises a plurality of optical elements:
a fixed first element that allows an image, which is most often virtual, of the observed object to be formed, which image is received by a second element;
movable second and third elements that allow the focal length of the zoom to be varied and a constant image back focus to be preserved when the focal length is varied; and
a fixed fourth element that receives the image formed by the preceding elements and forms it in the plane of a film or sensor, which may be a digital sensor for example.
In the case of a mechanically compensated zoom, the movements of the second and third elements follow independent complex laws in order to ensure the clearness of the focal plane whatever the value of the focal length.
The aperture of the zoom, which governs the amount of light received, is dictated by the diameter of the pupil. In conventional designs, the pupil, generally embodied by an iris of variable diameter, often has a fixed position and is then located either between the third and fourth elements or indeed in the interior of the fourth element. The exit pupil therefore occupies a fixed position, whereas the entrance pupil moves along the optical axis and has a diameter that varies depending on the focal length of the zoom.
Thus, during a change in focal length, i.e. when the third and second elements are moved, the useful diameters of the lenses located between the third element and the image plane remain constant whereas they vary for the lenses located between the third element and the front of the zoom.
Field aberrations such as coma, astigmatism or distortion depend on the position of the pupil and hence said position is also a degree of freedom for optimisation of the zoom.
Zooms of this type work over a wide range of focal lengths referred to as their “range” and often possess front optics of large diameter that are both tricky to produce, expensive and may increase the weight of and unbalance the zoom. Certain solutions or certain compromises already exist for decreasing the diameters, weights and cost of the optical components in a zoom. The following compromises may in particular be made: the “range” of the zoom may be decreased, focal lengths increased, aperture decreased, the aperture allowed to vary as a function of the position of the zoom in its range or “to ramp” but these compromises are necessary to the detriment of final performance.
For a given aperture number N, the diameter of the entrance pupil (D may be written as a function of the focal length F:N=F/Φ or Φ=F/N 
The size of an optic depends on the size of the pupil and on the distance to the pupil. When the pupil is far away, large fields lead to optics of significant dimensions.
In the case of very long focal lengths, the diameter of the front optics is governed by the aperture. It is optionally possible to tolerate “ramping” i.e. vignetting of the beam on the axis at long focal lengths in order to seek to minimise the diameter of the front optics.
In the case of systems of very short focal length, the diameter of the front optics mainly depends on the axial position of the entrance pupil of the objective and, of course, on the value of the field. According to the above relationship, the pupil diameter, which is small in this configuration, has relatively little influence.
An optical architecture that allows the entrance pupil to be as close as possible to the front lenses is therefore desirable. This may be obtained, for example, by moving the first element so that it remains in proximity to the entrance pupil of the system when the focal length of the system is varied. This solution has in particular been presented by the company “Carl Zeiss” in the publication entitled “Zoom lens design for projection optics”, Proceedings of SPIE volume 9626, 962617. It describes an example zoom that possesses a divergent movable first element, followed by a convergent movable second element and a fixed third element. The pupil of the system is fixed and is located at the entrance of the third element. By moving the first two elements, it has been observed that it is possible to preserve front optics of reasonable diameters.
Nevertheless, such a method turns out to be effective above all for zooms having a low range and quickly becomes ineffective for high ranges. Moreover, this solution is rarely envisaged for reasons of aesthetics and because of problems with seal tightness that this may sometimes occasion, the zoom not working at constant volume. Furthermore, the translation of the first element or of a module internal to this first element is generally used to focus on nearby objects because the movement laws are then independent of the focal length of the zoom.
Document US2013250160 entitled “Zoom lens with forward-located aperture stop” proposes a solution composed of four groups of lenses:                a divergent fixed first group;        a movable pupil located between the first and second groups;        a convergent movable second group;        a convergent movable third group and a fourth group that is very slightly convergent.        
This publication shows that the fact of positioning a movable pupil between the two first elements and of allowing it the freedom to follow its own movement law makes it possible to limit the diameters of the front and back optics. This method guarantees that the ratio of the diameters of the largest lens and of the smallest lens of the system does not exceed a ratio of two. Such a system is also effective only for zooms having reasonable zoom ranges, because although it allows the diameter of the front optics to be decreased, it leads to a significant increase in the diameters of the lenses of the second and third elements. For large focal-length ranges, the diameters of the lenses of the intermediate groups may become larger than those of the first element.
The second obstacle limiting the rays after the pupil is the stop. Architectures comprising two movable diaphragms have been described in the literature. Mention will be made, for example, of U.S. Pat. No. 3,918,798 entitled “Zoom lens having two diaphragms”, which describes an optical architecture of this type.
Another of the drawbacks of the latter type of solutions is that the movement of the pupil leads to changes in the aperture of the zoom, which are undesirable.